Mars moon obscures mighty Jupiter in rare satellite image (video)

Now you see it, now you don’t.

The Mars Express spacecraft witnessed a rare sight: Mars’ moon Deimos appears to be passing in front of Jupiter and its moons on Valentine’s Day (February 14). The event, called an eclipse, allows scientists to even more accurately calculate Deimos’ orbit around the Red Planet, a surprisingly difficult task.

In astronomy, an eclipse occurs when one celestial object moves in front of another, blocking the more distant object from view. In the recently released video, which consists of 80 frames captured by the Mars Express spacecraft’s high-definition stereo camera, Deimos first moves in front of points of light that are ocean moons Europa and Ganymede. It then crosses the white disk of Jupiter and finally two other large moons of Jupiter, the volcanic Io and the icy Callisto.

On the subject: Satellites of Mars: amazing photos of Phobos and Deimos

A still image from a frame of Deimos passing in front of Jupiter taken by the Mars Express spacecraft. (Image credit: ESA/DLR/FU Berlin – CC BY-SA 3.0 IGO, K.-D. Matz)

At that time, the Jupiter system was about 463 million miles (745 million kilometers) from Deimos, which was about 7.7 miles (12.4 km) wide.

A few weeks later, on March 30, Mars’ second moon, Phobos, passed in front of Deimos as seen by Mars Express.

For scientists studying the moons of Mars, all of these occultation phenomena are important to more accurately determine the orbits of Deimos and Phobos. Perhaps surprisingly, astronomers don’t know the exact shape and size of the orbits of Phobos and Deimos. But scientists know that gravitational forces between the moons and Mars cause the moons’ orbits to change.

Phobos, which is 14 miles (22.5 km) wide, orbits just 3,721 miles (5,989 kilometers) above the Red Planet — close enough to move around Mars faster than Mars orbits. The hypervelocity orbit causes the tidal bulge on Mars, caused by the gravity of Phobos, to lag behind the Moon. The gravity of the bulge then pulls on Phobos, causing Phobos to gradually slow down and slowly approach Mars. In 50 million years, Phobos will be so close to Mars, and the tidal forces will be so powerful that Phobos will fall apart, and the debris will either rain down on Mars or form a ring.

Deimos, on the other hand, has a wider orbit of 14,580 miles (23,460 km), resulting in its orbital period being slower than Mars’ rotation. This means that the tidal bulge on Mars, created by Deimos’ gravity, is being carried ahead of Deimos, causing the Moon’s orbital speed to increase as the bulge’s gravity pulls on Deimos. Consequently, its orbit becomes wider with time. Earth’s moon is slowly moving away from our planet due to the same type of interaction.

Although scientists understand the factors that change the orbits of Phobos and Deimos, attempts to accurately measure the orbits of the two satellites have run into difficulties. Moons are often lost in the bright light of Mars when viewed from Earth, so we have to rely on a spacecraft on the Red Planet to take measurements. One way to do this is to measure the duration of eclipses. The amount of time it took Deimos to hide Jupiter tells scientists exactly how far the Moon is from Mars.

The European Space Agency’s Mars Express mission has over the past 14 years observed the occultation of bodies in the solar system by the moons of Mars, using these events to constrain the orbits of Phobos and Deimos, improving our orbital measurements by 1.6–2 miles (1–2 miles). km).

The views of the eclipse also show how deformed both moons are; one theory is that they were captured from the nearby asteroid belt, while another theory is that they are fragments of a larger moon that was destroyed in an impact 2.7 billion years ago.

Follow Keith Cooper on Twitter @21stCenturySETI. Follow us on Twitter @Spacedotcom and on Facebook.

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